The invention relates to a mixing and kneading machine for continual compounding including a screw shaft rotating in a casing and simultaneously moving axially translationally and methods of implementing continual compounding by means such a machine.
Mixing and kneading machines of the kind presently involved are employed particularly for compounding bulk-flowable, plastic and/or pasty masses. For example, they serve processing viscous-plastic masses, homogenising and plasticizing plastics, admixing filler and strengthener additives as well as the production of starting materials for the food industry and the chemical/pharmaceutical industry often also involving continual venting, mixing and expansion being integrated. In some cases, mixing and kneading machines may also be employed as reactors.
The working member of the mixing and kneading machine is usually configured as a so-called screw which forwards the material for processing axially.
In conventional mixing and kneading machines the working member merely produces a rotational motion. In addition, mixing and kneading machines are also known in which the working member rotates whilst at the same time moving translationally. The motion profile of the working member is characterized particularly by the main shaft executing a sinusoidal motion overlying the rotation. This motion profile permits casing-mounting such fitted items as kneader pins or kneader teeth. For this purpose the screw is flighted to form discrete kneader vanes. The screw flights—kneader vanes—disposed on the main shaft and the casing-mounted fitted items interact in thus creating the desired shear/mixing and kneading functions in the various processing zones. Such mixing and kneading machines of the last-mentioned kind are known to persons skilled in the art under the trade name Buss KO-KNEADER®.
Mixing and kneading machines of the aforementioned kind are known in which the screw shaft diameter is up to 700 mm, it being particularly the screw shaft diameter that dictates the material throughput in each case. It is usually the case that the ratio of screw shaft outer diameter (Da) to screw shaft inner diameter (Di) is approximately 1.5 whilst the ratio of screw shaft outer diameter (Da) to stroke (translational motion component) (H) is approximately 6.7 and the ratio of pitch (axial spacing of the screw vanes) (T) to stroke (H) is around 2. Depending on the size of the mixing and kneading machine it is run at speeds of 5 to 500 rpm.
Mixing and kneading machines are usually engineered on the principle of geometric similarity. This exists when the ratios Da/Di, Da/H and T/H are constant, no matter what the magnitude.
The factors dictating how good the product being processed is dispersed, mixed and homogenized are the melt temperature, the residence time of the product in the processing space of the machine, the shear rate and the number of shear cycles in the screw channel/processing space filled with the melt.
As applies for many processes, the better the processing zones in sequence such as the infeed, melt, mix, dispersing and vent zones are harmonized as to delivery, shear rate level and fill, the better the product is mixed, dispersed and homogenized. In current state of the art in mixing and kneading machine technology the values usual to standard compounding are mean shear rates in the melt range from 15 l/s to 150 l/s and a mean product residence time over the full extent of the screw from 30 to 600 s.
In conventional mixing and kneading machines the mean shear rate is limited maximally by the rotational speed of the screw and the ratio Da/Di. But, increasing shear rates also result in higher values of the specific energy input which in turn can result in unacceptably high melt temperatures. In conjunction with a long mean residence time of the product in the mixing and kneading machine an excessively high shear rate may also result in deterioration of the product (thermal degradation or cross-linking) diminishing the quality.